Experts have reported that Eastern tent caterpillars are developing normally this spring and have just completed the egg hatch stage. Controlling the caterpillars is vital to area horse farms, as University of Kentucky research has strongly linked them with outbreaks of mare reproductive loss syndrome (MRLS), which can cause late-term foal losses, early-term fetal losses, and weak foals.

During 2001, when MRLS hit Central Kentucky particularly hard, an estimated 30% of the 2001-02 Thoroughbred foal crop was lost and the state suffered an economic cost of about $336 million due to losses suffered in all breeds of horses.

Epidemiological and field studies conducted by UK researchers demonstrated that MRLS was associated with unprecedented populations of Eastern tent caterpillars, wild black cherry trees, and waterfowl on Kentucky horse farms.

Eastern tent caterpillar

A series of studies over the next five years subsequently revealed that horses will inadvertently eat the caterpillars, and that the caterpillar hairs embed into the lining of the alimentary tract. Once that protective barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death from these alimentary tract bacteria is the hallmark of MRLS.

Experts recommend that horse farms scout for the telltale white tents in cherry and crabapple trees and err on the side of caution by keeping mares away from any caterpillar locations. Lee Townsend, PhD, UK College of Agriculture entomologist, offered several recommendations for controlling moderate to large caterpillar populations if horses cannot be moved to avoid possible exposure.

"Foliar sprays for caterpillar control can be made when tents are about the size of a baseball," he said. "Another option is the injection of trees with a systemic insecticide by commercial pesticide applicators or arborists. Regardless of the treatment used, it is important to revisit the sites in about five days to assess caterpillar activity."

Townsend said there is no indication of higher populations of the Eastern tent caterpillar this year. The first reported egg hatch in Central Kentucky occurred at the end of March in Scott County north of Lexington, he said.

Townsend anticipates the caterpillars will reach three-fourths to one inch in length by the middle of April, with full-grown larvae expected by the third week of April. By the end of April to beginning of May, caterpillars are likely to be dispersing from trees.

"These timeframes will vary depending on temperatures in the next few weeks," Townsend said. "A few warm days can cause them to grow quickly. As early spring insects, they are used to coping with temperature swings and rains."

Townsend said horse farms should hold off on spraying until caterpillars are a little more mature. "Poor early control in previous research has resulted in a recommendation against very early sprays against small caterpillars," he said.

If needed, control should target caterpillars while they are gathered together in the trees, Townsend said. Currently, small caterpillars are moving to feed on the leaves that have been appearing in trees and have built tents at branch and limb forks. The caterpillars will begin moving from branches to large limb angles within the trunk.

After a few weeks, the caterpillars will leave the trees where they've eaten the available foliage and search for food to complete their development. Once the caterpillars have reached these dispersing stages, controlling them becomes much more difficult, Townsend said.

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